From an early age I have been talking about and showing my girls wild and garden plants and trees. Now they enjoy looking for new ones and identifying them from books. My eldest daughter is 6 and on a recent school trip the teacher asked if anyone knew what sort of tree it was they had stopped at. (It was a Sycamore ) she shouted out the correct name.
By the end of the day the teacher was having regular consultations with my daughter on plant Id. She is now the class nature expert.
The key is to make it fun and talk about bushcraft and plant lore in a matter of fact way and use the correct names in normal conversation. They are like sponges and will take in any info you give them, even advanced botany and Latin names, only to regurgitate it at a later date word for word.
Take them with you.